Added Gradient method and tests.

Gradient computes finite differences of vector-valued function using finite difference
Made the gradient channels buffered to avoid contention
Fixed package name and added file headers
Added channels as receiver arguments and used assignment rather than subtraction in resetting xcopy
Made tests more robust to concurrent environment.
Fixed fmt statements with missing arguments
fixed Gradient documentation
Moved default step to be a function of the method
Changed step to only exist in Method
Merge pull request #4 from gonum/addgradient

Added Gradient method and tests.